A vacuum-assisted countergravity casting process using a gas permeable, self-supporting mold sealingly received in a vacuum chamber is described in such patents as the Chandley et al. U.S. Pat. Nos. 4,340,108 and 4,606,396. That countergravity casting process involves providing a mold having a porous, gas permeable upper mold member (cope) and a lower mold member (drag) sealingly engaged together at a parting plane, sealing the mouth of a vacuum housing to a surface of the mold such that a vacuum chamber formed in the housing confronts the gas permeable cope, immersing the bottom side of the drag in an underlying pool of melt, and evacuating the vacuum chamber to draw the melt upwardly through one or more ingate passages in the drag into one or more mold cavities formed between the cope and the drag.
Recent improvements in the vacuum-assisted countergravity casting process, represented by the Chandley U.S. Pat. Nos. 4,874,029 and 4,957,153 and the Aubin et al. U.S. Pat. No. 4,971,131 of common assignee herewith, have achieved substantial increases in the productivity and economies of the process. In these improved casting processes, mold cavity-forming means, such as one or more gas permeable molds (e.g., resin-bonded sand molds) or destructible patterns (e.g., polystyrene patterns) are surrounded in a mass of particulate mold material (e.g., binderless foundry sand) held within the open bottom container by establishment of a suitable negative differential pressure between the inside and the outside thereof. The particulate mass and molds/patterns are held in the container such that lower melt inlets of the molds/patterns are exposed at the open bottom end of the container for immersion in an underlying melt pool. The negative differential pressure between the inside and outside of the container is effective to draw the melt upwardly into the mold cavities as formed by the molds or patterns in the particulate mass. After the melt has solidified and the melt-filled container is moved to an unload station, the negative differential pressure is released to permit gravity-assisted discharge of the particulate mass, castings, and molds, if used, through the open bottom of the container.
While the aforementioned improved countergravity casting processes are preferably practiced using unbonded (i.e., binderless) particulates held within the container by the negative differential pressure, the processes may also be practiced using weakly bonded particulates in the manner taught in the Plant U.S. Pat. No. 4,848,439 wherein the particulates are bonded in-situ in the container by passing a gas/vapor curing agent through binder-coated particulates after they are introduced in the container about the mold cavity-forming means.
In practicing the aforementioned improved countergravity casting processes, the negative differential pressure is maintained between the inside and the outside of the container throughout the casting operation to hold the particulates about the molds/patterns in the container. Only when the melt-filled container is located at the demold station is the negative differential pressure released to allow the particulates, castings, and molds, if used, to discharge by gravity through the open bottom end of the container.
In view of the continuing desire for improvements in these vacuum-assisted countergravity casting processes, the provision of some means to hold the contents of the container from discharge through the open bottom end in the event of an interruption in the negative differential pressure would be welcomed. For example, such means would be welcomed as a way to avoid unexpected discharge of the container contents that might otherwise occur if the negative differential pressure is interrupted unexpectedly during the casting operation; e.g., as a result of malfunction of the vacuum pump evacuating the container. Moreover, such means would be welcomed as a way to impart flexibility to the process in that establishment of the negative differential pressure could be intentionally delayed/interrupted during the casting operation to achieve certain benefits. For example, establishment of the negative differential pressure could be delayed until after the mold and melt are engaged so as to avoid drawing any impurities, debris, and other foreign matter floating on the melt into the mold.
It is an object of the present invention to provide an improved countergravity casting apparatus and process of the type using a particulate mass held about mold cavity and inlet-forming means (e.g., one or more molds/patterns) in an open bottom container wherein the need to maintain a negative differential pressure between the inside and the outside of the container throughout the casting operation to hold particulates in the container is eliminated such that an unexpected interruption in the negative differential pressure will not result in discharge of the container contents during the casting operation.
It is another object of the present invention to provide an improved countergravity casting apparatus and process of the type using a particulate mass held about mold cavity and inlet-forming means in an open bottom container wherein the need to maintain a negative differential pressure between the inside and outside of the container throughout the casting operation to hold the particulates therein is eliminated such that establishment of the negative differential pressure can be intentionally delayed/interrupted during the casting operation to achieve certain process/product improvements.
It is another object of the invention to provide an improved countergravity casting apparatus and process of the type using a particulate mass held about a mold cavity and inlet-forming means in an open bottom container wherein the need to maintain a negative differential pressure between the inside and outside of the container throughout the casting operation is eliminated by providing a particulate mass which comprises a first portion of inherently unstable particulates (e.g., binderless particulates) disposed in the container about the mold cavity and inlet-forming means and a second supportive portion of bonded particulates disposed about the mold cavity and inlet-forming means beneath the first, inherently unstable portion for supporting the first portion and, if desired, the mold cavity and inlet-forming means as well as castings in the container during the casting operation, as may be necessary in the event of an intentional or unexpected interruption of the aforementioned negative differential pressure.